1. Field of the Invention
The invention relates to internal combustion engines and, more particularly, relates to a system and method for the simplified control of an exhaust gas recirculation (EGR) system of such an engine.
2. Discussion of the Related Art
Countries worldwide are implementing ever-stricter emission(s) standards for diesel and other internal combustion engines. Past and some current standards for oxides of nitrogen (NOx), hydrocarbon (HC), and particulate emissions have been met through various improvements to engine design, advancements in fuel injection equipment, and controls, etc. However, many of these techniques are incapable of meeting stricter emission standards that are being implemented or will soon be implemented by the United States and many other countries. Exhaust gas recirculation (EGR) is therefore becoming an increasingly-important weapon in the war against emissions.
EGR systems have been used for decades to reduce NOx emissions and, as now developed, have been successfully applied to modern gasoline engines to meet past and current emission regulations. Because of the tightening NOx standards for compression ignition (diesel) engines, EGR systems are currently being investigated for application to diesel engine emission systems for reduction of NOx. However, application of EGR systems to diesel engines presents several distinct challenges. For instance, the direct recirculation of hot exhaust gases to the air intake system of a diesel engine increases air intake manifold temperature, increasing hydrocarbon emissions and particulate levels due to deterioration of the combustion process. In addition, soot and other particulates in the EGR system accumulate in the aftercooler and other components of the engine""s intake and exhaust system, decreasing the effectiveness of those components and shortening their useful lives. Moreover, unlike in a throttled otto cycle engine, an unthrottled diesel engine often experiences an insufficient differential pressure across the EGR line to generate an EGR flow sufficient to obtain an optimal EGR mass fraction in the air/EGR mixture inducted into the engine.
Some of the problems associated with attempting to reduce emissions in a diesel engine through EGR, and proposed solutions to them, are discussed, e.g., in U.S. Pat. No. 5,440,880 to Ceynow, U.S. Pat. No. 5,806,308 to Khair, and U.S. Pat. No. 6,301,887 to Gorel. For instance, the Gorel patent discloses a so-called low pressure EGR system for a turbocharged diesel engine. The Gorel EGR system includes an exhaust particulate filter that is located downstream of and in fluid communication with the outlet of the turbocharger turbine for removing particulate matter from the exhaust gases. It also includes a low-pressure EGR line that extends from an inlet located within the main exhaust particulate filter to an outlet located upstream of the turbocharger compressor and downstream of the engine""s air filter. An EGR valve, an EGR cooler, and an EGR return are located in series within the EGR line. In addition, an EGR pick-up unit is located at the inlet of the EGR line within the main particulate filter. It has an internal particulate filter to remove particulates from the EGR stream.
According to the text of the Gorel patent, positioning the EGR pick-up unit upstream of the main soot filter places the EGR inlet in the high exhaust pressure environment created by the main particulate filter. The high exhaust backpressure and vacuum in the air intake are said to create sufficient differential pressure across the EGR line to allow for substantial EGR rates.
Solutions proposed by the Gorel patent and others solve some of the problems discussed above to the extent that it is now possible to implement a practical EGR system in a diesel engine on either an OEM or an aftermarket basis. However, available EGR designs useable in diesel or other engines still exhibit drawbacks and disadvantages that limit their market acceptance.
For instance, many EGR systems are so-called xe2x80x9cactivexe2x80x9d systems that employ an EGR valve that can be selectively controlled to vary EGR to optimize engine operation at existing load conditions. Depending on speed, load, and other parameters, the typical EGR valve is controlled to provide EGR on the range of 0 to 50% of total gas flow. Active control of the EGR valve requires knowledge or at least an indication of the percentage of the EGR in the mixture. This percentage is usually determined on a mass fraction basis, i.e., on the basis of the fraction of the mass of the EGR of the total mass of the EGR/air mixture. EGR mass fraction historically has been determined directly from direct mass flow measurements obtained both downstream and upstream of the venturi or other device used to draw EGR into the incoming air stream. This technique requires the incorporation of at least two mass flow sensors in the EGR systemxe2x80x94one measuring EGR mass flow and one or two measuring the fresh air mass flow and/or the mixture mass flow. These mass flow sensors are relatively expensive and unreliable.
In addition, prior attempts to increase the back pressure in an EGR system sufficiently to obtain EGR percentages of levels desired at low load have proven only partially effective and/or have required the incorporation of rather complex, expensive back pressure generation devices into the EGR system. For example, the pick up unit proposed in the Gore patent is much more complex and expensive than a simple venturi. Finally, the particulate trap of these systems tends to become clogged with particulates at low operating temperatures.
Proposals have been made to eliminate the need to sense mass flows directly in an EGR system, hence addressing some of the problems discussed above. For instance, U.S. Pat. No. 6,035,639 to Kolmanovsky proposes the estimation of gas flow in a turbocharged diesel engine by estimating an EGR flow value as a function of a measured intake manifold pressure, a measured exhaust manifold pressure, a measured position of an EGR valve, and a measured EGR temperature. From this calculation, the system generates an intake air flow value (MAF). The MAF value then is used to control the position of the EGR valve. This system, while lacking the need for direct mass flow measurements, does not calculate EGR mass fraction or even a value indicative of it. It is also relatively complex, requiring the obtainment of at least the following items of information to control the EGR value:
Intake manifold pressure (MAP);
Exhaust manifold pressure (EXMP);
EGR valve setting;
EGR temperature; and
Intake air temperature.
Other systems relying in part on temperature based measurements rather than directly on mass flow measurements suffer similar deficiencies. Systems of this type are disclosed, e.g., in U.S. Pat. No. 5,273,019 to Matthews et al.; U.S. Pat. No. 5,520,161 to Klopp; and U.S. Pat. No. 5,601,068 to Nozaki. None of them calculate EGR mass fraction using only temperature measurements to obtain EGR related information.
Some of these problems, and particularly the mass flow sensor problem, extend beyond diesel engine EGR systems to the more traditional EGR systems for otto cycle engines as well. The need therefore has arisen to provide a simplified method and apparatus for determining a value indicative of the EGR mass fraction in an active EGR control system. The need additionally has arisen to provide a simple, reliable, effective structure and technique for directing EGR into an air induction system of an internal combustion engine. The need additionally has arisen to provide an EGR system with a reliable, low cost particulate trap whose effectiveness does not degrade at low temperatures.
In accordance with a preferred aspect of the invention, an improved method and system for controlling an EGR system includes measuring a temperature of the EGR exhaust gases, a temperature of the supply air, and a temperature of an EGR/mixture being directed toward the engine""s air intake. The method additionally comprises calculating, based on the measuring step, a parameter indicative of a mass fraction of exhaust gases (mfegr) in the mixture; and adjusting an engine operating parameter based on the calculating step. The calculating step preferably is performed solely on the basis of the measuring step. It may comprise calculating mfegr, preferably by solving the equation:       mf    egr    =                    T        mix            -              T        air                            T        egr            -              T        air            
The system preferably can be electronically controlled to selectively vary the EGR ratio as a function of engine operating conditions, and this control can be accomplished independently of the control the engine""s fuel and air management systems. Automatic controls can be added to the system to adjust the EGR flow as a function of engine operating condition and to automatically regenerate the particulate trap. The preferred embodiment provides EGR in the range of 5 to 20% of total gas flow at full load and 0 to 50% at low load. The preferred embodiment utilizes an exhaust particulate filter for the EGR gas only, an EGR cooler after the filter, a second stage filter after the EGR cooler, and a fixed venturi in the intake air system. The preferred venturi, which is also usable in other systems, includes an air inlet, a mixed gas outlet, a throat located between the air inlet and the mixed gas outlet, and an exhaust gas inlet located in the vicinity of the throat. The venturi is configured to generate a suction pressure of between 0.04 and 0.08 atmospheres and has a throat Mach Number of between 0.1 and 0.5 at an air flow rate of between about 40 lb/min and 80 lb/min. More preferably, the venturi is configured to generate a suction pressure of about 0.06 atmospheres and having a throat Mach Number of about 0.3 at an airflow rate through the venturi of about 60 lb/min.
The particulate trap of the preferred embodiment, which is usable in other applications as well, may comprise an actively regenerated catalytic particulate trap that can be actively regenerated by injecting a fuel into the particulate trap. In this case, active regeneration is performed whenever a pressure differential thereacross exceeds a designated value. For maximum reduction of particulates, the size of the oxidizing particulate filter can be increased to accommodate the entire exhaust flow rather that only the part to be returned to the engine.
The end result is a variety of practical tools that can be applied individually or combined to control the exhaust emissions of internal combustion engines with particular emphasis on piston engines fuelled by diesel fuel, natural gas or a combination of both fuels.
Other aspects and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications could be made within the scope of the present invention without departing from the spirit thereof, and the invention includes.